The present invention is directed to a pressure sensor for measuring fluid pressure, and in particular to a pressure sensor including a housing having a first body member and a second body member and a radially tensioned flexible diaphragm disposed between the first and second body members, wherein the material that forms the first and second body members is matched to the material that forms the diaphragm such that the coefficients of thermal expansion of the materials are compatible with one another.
Pressure sensors have previously included body members and a metal diaphragm sandwiched between the body members. The metal diaphragm has been radially tensioned by radially expanding the diameter of the diaphragm as disclosed in U.S. Pat. No. 6,019,002. The metal diaphragm has also been radially tensioned by forming the diaphragm from a precipitation hardenable metal and subjecting the precipitation hardenable metal in an annealed condition (A) to an aging treatment at high temperatures ranging from about 500 degrees to 600 degrees Celsius (C.) for one hour whereby the precipitation hardenable material contracts and radially tensions the diaphragm as disclosed in U.S. Pat. No. 4,158,311.
It has been found that if the coefficient of thermal expansion (Tc) of the metal material that forms the body members of the sensor does not sufficiently closely match the coefficient of thermal expansion of the metal material that forms the diaphragm, the diaphragm can experience a high radial tensile stress during heat treating that can exceed the yield stress of the material that forms the body members. Consequently, the body members will yield and thereby release the radial tension in the diaphragm that was created by the heat treatment process, resulting in zero net radial tension in the diaphragm at ambient temperatures. In addition, during use, a sufficient mismatch between the coefficients of thermal expansion of the material that forms the body members, and of the material that forms the diaphragm, causes a change in the diaphragm radial tensile stress as the temperature of the sensor changes, thereby causing a direct change or error in the pressure reading at Span (at full scale pressure). The metal diaphragm may also be moved by magnetic fields, as opposed to changes in pressure, thereby providing an inaccurate pressure reading. The present invention overcomes these problems in the prior art.
A pressure sensor for measuring fluid pressure. The pressure sensor includes a housing having a first generally concave metal body member and a second generally concave metal body member. A radially tensioned flexible metal diaphragm is disposed between the first body member and the second body member. The first body member and the diaphragm form a first fluid chamber and the second body member and the diaphragm form a second fluid chamber. The first and second body members are formed from a first material having a first coefficient of thermal expansion, and the diaphragm is formed from a second material having a second coefficient of thermal expansion. The first coefficient of thermal expansion of the body member metal is not greater than the second coefficient of thermal expansion of the diaphragm metal more than by approximately 0.0000015 inch/inch/xc2x0 F. The second coefficient of thermal expansion of the second material that forms the diaphragm is preferably approximately 0.0000060 inch/inch/xc2x0 F. The first coefficient of thermal expansion of the first material that forms the body members is preferably in the range of approximately 0.0000056 inch/inch/xc2x0 F. to 0.0000064 inch/inch/xc2x0 F. The first body member and the second body member may be formed from a ferromagnetic material such that the first and second body members shield the diaphragm from magnetic fields which may otherwise cause movement of the diaphragm which results in inaccurate measurement of fluid pressure.